"There are a thousand hacking at the branches of evil to one who is striking at the root." - Henry David Thoreau

To simplify it for you: guns = branches, motivation to kill = roots.

To use a hyperbole: atomic bombs don't kill people, people kill people. Let's give every law abiding citizen an atomic bomb.

Now clearly guns are not as destructive as atomic bomb. But they are destructive nonetheless. Nobody is a criminal until they decide to be one. There is no way of telling who will become a criminal. Precogs (from the Minority Report) have not been invented yet.